Iran was plunged deeper into crisis on a day of religious mourning yesterday as political tensions erupted into violence that left at least nine dead, including a nephew of the main opposition leader.

Mayhem unfolded in Tehran after a brutal crackdown in which security forces fired on protesters gathered on Ashura, one of the holiest days in the Shia calendar. The shootings killed at least four people, with another said to have died from head injuries after being beaten by police. Among the dead was Ali Mousavi, a nephew of Mir Hossein Mousavi, leader of the reformist movement. He was reported to have been shot through the heart.

Demonstrators – many chanting slogans against Iran's supreme leader, Ayatollah Ali Khamenei – retaliated by attacking members of the security forces, in some cases beating them with their own batons. Police cars were set on fire and photographs appeared to show riot officers retreating under a hail of stones.

A further four people were killed and many others injured in the northern city of Tabriz, according to reformist websites. Clashes were also reported in several other cities, including Isfahan, Shiraz, Arak, Mashhad, Babol and Najafabad.

The accounts could not be confirmed because of restrictions on the foreign media in Iran. The state broadcaster Irib quoted a senior police official as saying 300 people had been arrested. Last night opposition websites reported that clashes were continuing in central Tehran.

It was Iran's worst outbreak of violence since last June's disputed election, which the opposition claims President Mahmoud Ahmadinejad stole through mass fraud. But yesterday's events differed from previous clashes in a way that seemed to herald further turmoil to come.

Last night an opposition leader and cleric, Mehdi Karoubi, criticised the country's rulers over the deaths. "What has happened to this religious system that it orders the killing of innocent people during the holy day of Ashura?", Karoubi, who came fourth in the election, said in a statement, the Jaras website reported.

By using lethal force on a day meant to honour one of Shia Islam's holiest figures, Imam Hossein – seen as a martyr in the fight against oppression – the regime may have undermined its claim to uphold Iran's religious traditions. The violent response of the protesters to the security forces was also unprecedented and suggested that many are becoming fearless in the face of state repression. While many demonstrators in the post-election protests covered their faces, footage from videos shown yesterday on YouTube showed most people with their faces exposed.